标题： 旋涡星系NGC 0628、NGC 3627和NGC 7331的射电光谱指数图像 显示英文标题

标题： Radio spectral index images of the spiral galaxies NGC 0628, NGC 3627, and NGC 7331

摘要： 为了理解星系中的宇宙射线传播机制及其与恒星形成区域的相关性，我们将三个螺旋星系的分辨射电光谱指数与其红外分布进行了比较。 我们提供了银河系NGC 0628、NGC 3627和NGC 7331的新低频射电连续谱观测数据，观测频率为327 MHz，使用甚大天线阵列。 我们补充了在1.4 GHz处的敏感档案观测数据，并研究了这些螺旋星系盘内射电光谱指数的变化。 我们还使用70$\mu$m的斯皮策观测数据比较了光谱指数分布和红外分布。 我们发现，在这些星系中，非热光谱指数与射电亮度呈负相关。 明亮区域，如NGC 3627的棒状结构或NGC 7331的核周围区域，相对于其下的盘面具有更平坦的光谱。 因此，观察到这些星系中心距离增加时，光谱指数系统性变陡。 此外，通过将射电图像与斯皮策卫星的70$\mu$m图像进行比较，我们发现射电光谱指数与红外亮度之间存在类似的负相关关系，这符合基于射电连续谱与红外辐射局部相关性的预期。 我们的结果支持这样一个观点，即在强烈的恒星形成区域，电子扩散必须是高效的。 观测到的射电亮度与光谱指数之间的负相关可能意味着这些区域的宇宙射线密度和磁场强度明显高于其周围环境。 显示英文摘要

摘要： In order to understand the cosmic ray propagation mechanism in galaxies, and its correlation with the sites of star formation, we compare the spatially resolved radio spectral index of three spiral galaxies with their IR distribution. We present new low-frequency radio continuum observations of the galaxies NGC 0628, NGC 3627, and NGC 7331, taken at 327 MHz with the Very Large Array. We complemented our data set with sensitive archival observations at 1.4 GHz and we studied the variations of the radio spectral index within the disks of these spiral galaxies. We also compared the spectral index distribution and the IR distribution, using 70 $\mu$m Spitzer observations. We found that in these galaxies the non-thermal spectral index is anticorrelated with the radio brightness. Bright regions, like the bar in NGC 3627 or the circumnuclear region in NGC 7331, are characterized by a flatter spectrum with respect to the underlying disk. Therefore, a systematic steepening of the spectral index with the increasing distance from the center of these galaxies is observed. Furthermore, by comparing the radio images with the 70 $\mu$m images of the Spitzer satellite we found that a similar anticorrelation exists between the radio spectral index and the infrared brightness, as expected on the basis of the local correlation between the radio continuum and the infrared emission. Our results support the idea that in regions of intense star formation the electron diffusion must be efficient. The observed anticorrelation between radio brightness and spectral index, may imply that the cosmic ray density and the magnetic field strength are significantly higher in these regions than in their surroundings.